This invention relates to apparatus for providing depolarized light.
In a conventional optical time domain reflectometer (OTDR), light emitted from a laser diode is applied to a directional coupler. In the bar state of the coupler, the light from the laser diode is launched into an optical fiber under test. In the cross state of the coupler, backscattered light from the fiber under test is directed towards a detector, such as an avalanche photodiode.
Normally, the light emitted by a laser diode is linearly polarized, and therefore the backscattered light received by the detector also is polarized, but the relative amplitudes of the two polarization eigen states will vary due to small variations in the birefringence of the fiber under test as a function of its length. If the detector is sensitive to the polarization state of light that it receives, or the directional coupler has different transmissions in the cross state for the two polarization states, the signal provided by the detector is modulated in response to the variations in the birefringence of the fiber under test. The effect whereby the output signal provided by the detector is modulated in this manner is known as polarization ripple, and is generally undesirable. Polarization ripple can be eliminated by reducing the polarization sensitivity of the detector and making the directional coupler polarization insensitive in the cross state, or depolarizing the launched laser light, or a combination of these measures.
Light that is polarized can be characterized in terms of the relative amplitudes of the two polarization eigen states and the phase coherence between the two polarization eigen states. If light is linearly polarized, all the energy in the optical field is in one of the polarization states, and the amplitude of the other polarization state is zero. For light that is partially polarized, the amplitudes of the two polarization states are both non-zero, and there is partial phase coherence between the two states. For light that is depolarized, the amplitudes of the two polarization eigen states are equal and there is no phase coherence between the two states.
When the amplitude of the optical output provided by a laser diode is plotted as a function of wavelength, the curve is composed of a series of narrow peaks, or longitudinal modes, at wavelengths that are integral sub-multiples of twice the length of the laser diode channel. The Fourier transform of the amplitude curve is approximately Gaussian in form, and the width of the Fourier transform curve at the amplitude that is (1/e.sup.2) times its maximum amplitude is referred to as the coherence time associated with the longitudinal modes. The coherence time associated with the longitudinal modes depends inversely on the width of the longitudinal modes.
The longitudinal modes have an envelope that is approximately Gaussian in form. The autocorrelation function of the light source is the Fourier transform of the envelope of the longitudinal modes and is composed of closely spaced peaks within an envelope formed by the Fourier transform of the amplitude curve. The width of one of these peaks at the amplitude that is (1/e.sup.2) times the maximum amplitude of the peak is referred to as the coherence time associated with the autocorrelation function, and depends inversely on the bandwidth of the laser diode. The spacing of the peaks of the autocorrelation function depends inversely on the spacing of the longitudinal modes.
A typical laser diode used in OTDRs emits light with a bandwidth of 2 nm and a coherence time associated with the autocorrelation function of about 3 ps.
If light in one of the polarization states is retarded relative to light in the other state by a time that is greater than the coherence time associated with the longitudinal modes of the laser diode, phase coherence between the two polarization states is destroyed. Alternatively, phase coherence is destroyed if light in one state is retarded relative to light in the other by a time that is greater than the coherence time associated with the autocorrelation function of the laser diode and is approximately equal to (n+1/2) times the period of the autocorrelation function, where n is an integer.
When partially polarized light is propagated through birefringent material, light in one of the polarization states is retarded with respect to light in the other polarization state. Unless the retardation exceeds the coherence length (the product of the propagation velocity and the coherence time associated with the autocorrelation function of the light source), phase coherence can be restored.
The polarization sensitivity of an OTDR is proportional to the product of the degree of polarization of the light launched into the fiber under test and the degree of sensitivity of the detector to the state of polarization of the backscattered light received from the fiber under test. The polarization of the launched light can be reduced by placing a depolarizer between the laser diode and the directional coupler, but some forms of couplers have different insertion losses for different polarization states so that the light launched into the fiber under test will be somewhat polarized even if the light received at the directional coupler is completely depolarized. M. P. Gold, "Design of a Long-Range Single-Mode OTDR", IEEE J. of Lightwave Tech., LT-3, 1, 39 (1985) describes the problem of polarization sensitivity, and suggests that the problem may be alleviated by manipulating the fiber under test to scramble the polarization state of light being propagated in the fiber. However, Gold states that it is more effective to avoid polarization sensitivity in the return path of the OTDR.
T. Horiguchi, M. Nakazawa, M. Tokuda and N. Uchida, "An Acoustooptical Directional Coupler for an Optical Time-Domain Reflectometer", IEEE J. Lightwave Tech., LT-2, 2, 108 (1984) describes the problem of polarization dependency in the context of an acoustooptical direction coupler.
E. Brinkmeyer and J. Streckert, "Reduction of Polarization Sensitivity of Optical-Time Domain Reflectometers for Single-Mode Fibers", IEEE J. Lightwave Tech., LT-4, 5, 513 (1986) discusses use of a highly birefringent launching fiber that acts as a depolarizer.
T. Horiguchi, K. Suzuki, N. Shibata and S. Seikai, "Birefringent Launching Fibers for Reducing Backscattered Power Fluctuations in Polarization-Sensitive Optical-Time-Domain Reflectometers", J. Opt. Soc. Am., 2, 10, 1698 (1985) also describes use of a birefringent launching fiber for reducing polarization dependence of an OTDR.
T. Horiguchi, K. Suzuki, M. Shibata, M. Nakazawa and S. Seikai, "A Novel Technique for Reducing Polarization Noise in Optical-Time Domain Reflectometers for Single-Mode Fibers", IEEE J. Lightwave Tech., LT-3, 4, 901 (1985) describes use of a polarization-holding launching fiber to depolarize light launched into the fiber under test and thereby reduce polarization dependence of an OTDR.
B. G. Koehler and J. E. Bowers, "In-line single-mode fiber polarization controllers at 1.55, 1.30, and 0.63 .mu.m", Applied Optics, Vol. 24, No. 3, 349 (1985) describes a polarization controller made from two coils of single mode optical fiber connected in series. Polarized light is introduced into the polarization controller, and by adjusting the relative orientation of the coils, the relative amplitudes of the two polarization eigen states of light leaving the controller can be varied.